Incompatibility with mathtools package

[IsraelBuitronD]

If we add \usepackage{mathtools} into the setup file (e.g. my-thesis-setup.tex), then we will have some compilation error.

Suppose you need to use mathtools package to have available math commands like \mathbb, \mathcal, etc., and a chuck of code like:

\begin{equation}
  \mathbb{F} = \emptyset
\end{equation}

you will have an error message, like this:

Undefined control sequence. [...\mathbb]

[JanWaldhorn]

If you include the other packages instead it works without problems

\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}

[mcomis]

I had the exact same issue. Was able to fix it by protecting line 435 in cleanthesis.sty, i.e., by replacing it with
\protect\settowidth{\lensectionnumber}{\textbf{\thesection}\quad}